Generally, the present invention relates to the field of surge test equipment. More specifically it relates to methods and apparatus to automatically determine the existence and extent of a fault in an electrical coil from a surge test.
The invention focuses on several needs of users of surge test equipment. These users include both manufacturers--who test their products before shipping--and users--who test their equipment in the field as part of maintenance procedures. From an understanding of both of these perspectives, the present invention addresses the needs of these persons and addresses limitations found in existing surge test equipment. One such need is the desire of those performing the test to rapidly ascertain the integrity of the item being tested. This is of particular concern in the surge test environment because production line testing may need to be accomplished very quickly. Another limitation of existing surge test apparatus is the inherent difficulty in accurately assessing the existence of a fault in the equipment being tested. Because traditional techniques often have been based upon a visual comparison of waveforms which are the reaction of the equipment to the surge, such visual comparison has been difficult to adapt for an automatic determination. This difficulty has perhaps been underscored by the fact that in spite of increasingly sophisticated analysis means becoming available, the vast majority of surge test equipment is still based on visual analysis by an operator. Since unacceptable fault levels in the equipment being tested are sometimes hard to detect visually, the operators would ideally have some degree of skill in analyzing the surge test response waveforms. This is inconsistent, however, with the need to have such waveforms reviewed in a highly repetitive fashion on an assembly line as frequently as every few seconds. Naturally such a method also introduces the possibility of human error and its associated limitations. While smaller and smaller tolerances have been demanded, the practical limitations inherent to a visual technique have been difficult to overcome. Although several efforts have been made to automate the determination of the existence of a fault in the equipment being tested, these efforts have met with varying degrees of success and have often proved not to provide as accurate a result as even the visual testing traditionally done. The present invention not only addresses each of these needs but several others.
Two of the additional aspects focused on in the present invention are particularly noteworthy. First, the nature of a surge test is such that the surge imposed upon the equipment to be tested actually weakens or, in extreme cases, can create a fault in that equipment. Although this aspect has been well known, traditional surge testing has not automatically limited the stress to which the equipment is subjected. In fact, through the existence of industry standards such as National Equipment Manufacturers Association Standard 1-12.05, repetitive surge testing has been widely supported. The present invention addresses this aspect by providing methods and test apparatus which automatically minimize the stress to which the equipment is subjected. Second, traditional surge testing has been through simultaneous comparison of the tested coil with a coil which is assumed to be acceptable--that is, a standard coil. In traditional techniques this usually has involved repeatedly subjecting the standard coil to identical surges as each different test item is analyzed. Not only does this weaken the standard coil but it is attended with other practical and power consumption concerns. Since the traditional technique of comparison testing is not an exact science, it has also been necessary to experimentally ascertain the threshold amount of change in the response to the surge at which a "fault" condition is determined to exist. This has been done through intentionally faulting an acceptable coil in the smallest possible way and observing the amount of change so induced. Obviously this technique has several undesirable features. The present invention addresses each of these aspects and the aforementioned aspects in one invention.
As background to surge testing in general, it should be understood that the technique of subjecting an electrical coil to a voltage surge is well known having been disclosed at least by 1943 in an article by C. M. Faust and N. Rohats entitled "Insulation Testing of Electrical Windings" (Trans. AIEE Vol. 62, pp. 203-06). Basically the technique involves subjecting an electrical coil, such as is frequently found in an electrical motor, to a sharp, high voltage pulse and then allowing this pulse to oscillate or "ring" within the coil. This ringing produces decaying oscillations which may vary in several ways if there is any fault within the winding. One such type of fault is a breakdown in the insulation between adjacent coils--a turn-to-turn fault. Such a fault would change the inductance and capacitance characteristics of the coil and would thus be seen in the resulting waveform. Of particular importance is the need to subject the coil to one or more high voltage pulses in order to detect an incipient fault in such insulation. The fact that small breakdowns may not be visible until several surges have been accomplished is one reason for using repetitive surges for testing. The basic techniques involved are well known and have been the subject of numerous inventive efforts. An example of the types of waveforms occurring for the various possible fault scenarios is contained in several articles by the assignee including: "Winding Fault Diagnosis by Surge Comparison" presented at the Fourteenth Electrical/Electronic Insulation Conference and "Surge Test Methods for Rotating Machines" as published by the IEEE.
While several inventive efforts have been directed to automating the technique of surge test analysis, most all of the automatic testers have been based upon the technique of comparing voltage levels of the test with those of a standard coil. Although envelope decay rate has also been used, it is almost always implemented in conjunction with voltage levels. The voltage level criterion has met with varying degrees of success and has not always resulted in more accurate determinations than were visually possible. U.S. Pat. No. 3,659,197 as assigned to General Electric Corporation presents an automatic testing device based on voltage comparisons which also allows visual analysis capabilities. That General Electric patent--through providing one technique for visual analysis and another technique for automatic analysis--also alludes to the inherent difficulties those skilled in the art have faced in attempting to develop a technique acceptable for automatic analysis. Another example of the prevalence of the use of voltage difference determinations as the criterion, is shown in U.S. Pat. No. 3,869,664 as assigned to Avtron Manufacturing, Inc., and its related patents. Although digital techniques have been available for some time, the focus by those skilled in the art on absolute voltage level criteria has become an impediment to the adaptation of automated techniques to the surge testing field. Even though absolute voltage criterion can be readily adapted to digital analysis, the potential for bad data points and its resulting in false indications has been undesirable.
Since the present invention, in its preferred embodiment, is based upon well known computer integration and sampling techniques, at first glance it would seem that those skilled in the art would have had no trouble implementing these techniques to their field. This would seem especially true because there has been a long-felt need for accurate and reliable and automatic assessment of surge test results. The limitation that those skilled in art faced was that they simply failed to realize that the problem lay in properly choosing the detection criterion. They did not appreciate that the voltage difference criterion was at the root of the problem. Although the variety of patents in the field of automatic surge test equipment and the broad range of dates of these inventive efforts show that substantial attempts were made to automate the equipment, the fact that the traditional technique of visually detecting a fault still remains as the preferred technique shows that those attempts did not fill the need of surge test users. They simply did not understand that the effort necessary in this regard was not in refinement of the systems involved but rather was in development of a proper detection criterion. The broad acceptance of a voltage difference criterion by those skilled in the art of producing automated surge test apparatus basically led by teachings away from the technical direction of the present invention. It is also noteworthy that it was even a surprise to the inventor that the development of an area-based analysis resulted in not only one standard which was consistent across a broad range of motor types and characteristics, but also that utilization of such a technique lent itself so well to the data analysis capabilities described herein.
The present invention recognizes and addresses each of these concerns and overcomes the limitations perceived by those skilled in the art by presenting methods and apparatus which, among other aspects, allow for digital processing and which overcome the difficulties in implementing such processing to the surge test field. The techniques and devices utilized in the present invention result in more accurate testing, in automatic testing and in testing which is more suitable from both the user's and manufacturer's perspectives.